Hybrid vehicles are known which have an internal combustion engine and one (or more) electric motor(s) for their propulsion. To decelerate such motor vehicles, the electric motor can be operated as a generator and the current generated stored in an accumulator. Decelerating the motor vehicle with the electric motor operated as a generator is called recuperative braking. The electric motor can generally be regarded as an electric machine, for example it can also be a generator.
To achieve a specific deceleration, a braking effect of the hydraulic vehicle brake system must be reduced by the deceleration effect of the electric machine operated as a generator. In simple systems, a vehicle driver must compensate for the braking effect of the electric machine operated as a generator or the deceleration of the motor vehicle is increased via the braking effect of the vehicle brake system by the deceleration effect of the electric machine operated as a generator.
In more complex systems, an attempt is made to compensate automatically for the deceleration effect of the electric drive machine operated as a generator by reducing the braking effect of the hydraulic vehicle brake system, wherein the compensation succeeds to a greater or lesser extent. Compensation for the deceleration effect of the electric machine operated as a generator by reducing the braking effect of the vehicle brake system is called covering. Efforts are focused on not only compensating to a greater or lesser extent for the deceleration effect of the electric machine operated as a generator, but also on ensuring that the pedal feel at a brake pedal (or handbrake lever) remains as unchanged as possible i.e. a vehicle driver where possible does not perceive or perceives very slightly that the motor vehicle is being decelerated to a greater or lesser extent by the electric machine operated as a generator and the braking effect of the vehicle brake system is reduced accordingly.
One difficulty in such covering is that the deceleration effect of the electric machine operated as a generator depends on various circumstances and changes during braking. One such circumstance is that the braking effect of the electric machine operated as a generator diminishes as the vehicle speed diminishes, down to zero when stopped. A charge state of the accumulator is another such circumstance: if the accumulator is fully charged and therefore cannot receive any electric current, generator operation of the electric machine is not possible and the motor vehicle must be braked conventionally exclusively by the hydraulic vehicle brake system. Shift processes also cause sudden changes in the deceleration effect of the electric machine operated as a generator if this is decoupled on decoupling the vehicle wheels.
Publication DE 10 2008 003 664 A1 discloses a hydraulic vehicle brake system with a slip control for a hybrid vehicle. In service braking, a brake circuit is isolated hydraulically from a master brake cylinder by closing an isolating valve and a wheel brake pressure is generated with a hydraulic pump, and as in slip control, regulated by brake pressure build-up and brake pressure reduction valves. Regulation in the sense of the disclosure also means control. The wheel brake pressure is reduced by the amount corresponding to the deceleration by an electric machine of the motor vehicle operated as a generator, wherein a total deceleration of the motor vehicle with the electric machine operated as a generator and the hydraulic vehicle brake system should correspond as precisely as possible to a deceleration which would be achieved by braking exclusively by activating the vehicle brake system with a master brake cylinder and without the electric machine in generator operation. An activation travel and/or activation force of the master brake cylinder and/or a master brake cylinder pressure constitutes a nominal value for the braking effect. A vehicle driver during braking should not perceive that part of the deceleration is achieved by the electric machine operated as a generator. A changing deceleration effect of the electric machine operated as a generator during braking should be as imperceptible as possible. A second brake circuit is activated in the conventional manner with the master brake cylinder in the known vehicle brake system, where no regulation intervention occurs on braking to compensate for the deceleration effect of the electric machine operated as a generator.
Publication DE 102 33 838 A1 discloses a master brake cylinder for an electro-hydraulic i.e. external energy vehicle brake system. The master brake cylinder has two pistons for a brake circuit, one of which as a so-called rod piston is mechanically connected with a brake pedal. A carrier device carries a second piston which pressurizes the same brake circuit after a pre-specified piston travel of the first piston. Until the second piston moves with the first piston, an effective piston surface of the first piston is smaller than the effective piston surface of both pistons together when, after covering the pre-specified piston travel, both pistons move. The smaller effective piston surface causes a lower activation force at a given master brake cylinder pressure. As a result the pedal force rise is reduced when the master brake cylinder is isolated hydraulically from the vehicle brake system by closure of an isolation valve for an external energy brake. The vehicle brake system is not braked with the master brake cylinder, the master brake cylinder serves as a nominal value emitter for a braking force to be set. A hydro-accumulator communicating with the master brake cylinder receives brake fluid which is displaced from the master brake cylinder on activation.
The movement travel of the first piston until carrying the second piston is dimensioned such that normally in service braking only the first piston is moved. The second piston which enlarges the effective piston surface is provided for auxiliary braking in the event of a fault in the external energy vehicle brake system. The second piston enlarges a brake fluid volume displaced from the master brake cylinder per piston stroke to activate the vehicle brake system with the master brake cylinder. The second piston is designed such that, on auxiliary braking in the event of a fault in the external energy vehicle brake system, sufficient brake fluid is displaced from the master brake cylinder to activate the vehicle brake system and that an activation travel of the master brake cylinder is not extended too far.